The present invention relates to a display element using a display material capable of causing deposition, dissolution or color change based on, for example, electrochemical reduction or oxidation.
With recent dissemination of network, there are increasing trends in distributing various documents in a form of so-called electronic document, instead of in a conventional form of printed matter. Also books and magazines are becoming more frequently distributed through electronic publishing.
A conventional way for viewing these information is such as reading the information on cathode ray tubes (CRTs) or on liquid crystal displays (LCDs). It has, however, been pointed out that these light-emitting-type displays would make the users heavily tired due to reasons related to human engineering when they are watched for a long duration of time, and that they are not suitable for reading for a long duration of time. An additional disadvantage is that sites of reading are limited only at around places where computers with display devices are installed.
While recent dissemination of laptop computers made it possible to use some of them as mobile displays, they are still not suitable for reading for a long duration of time because they are intrinsically of light-emitting type and problems in power consumption remain unsolved.
Reflection-type, liquid-crystal displays recently developed seem to be driven at low power consumption, but a reflectivity of only as high as 30% or around is attainable for the blank display (white display). This results in only a far poorer visibility as compared with that of printing on paper, is more causative of fatigue, and is far from ensuring comfortable reading for a long duration of time.
To solve these problems, recent development has been directed to so-called, paper-like display or electronic paper. This type of medium usually develops color based on electrophoretic migration of colorant particles between electrodes, or rotation of dichroic particles in an electric field. These methods, however, have drawbacks in that they can attain only a poor contrast because the light is undesirably absorbed by gaps between the particles, and this makes it impossible to achieve a practical write speed (within one second) unless drive voltage is raised to as high as 100 V or above.
Electrochromic displays (ECD) are superior to any other systems described in the above in terms of contrast, and have already been put into practical use for auto-dimming glass or displays for watches and clocks.
The conventional configuration intended for use in displays such as the electronic paper always needed matrix driving. The conventional displays based on electro-chemical reaction, however, could not adopt the simple matrix system due to their large energy consumption for the driving, and instead the active matrix system have inevitably been adopted. This has consequently raised price of the display devices, so that there have been strong demands for the devices which can be driven based on the simple matrix system.
The present inventors found out an ion-conduction-type display element capable of developing color(s) based on a simple matrix system.
Configuration and functions of such a display element are as described below.
That is, ions will migrate towards either electrode with the aid of electric field energy at an electrode-intersecting area where the potential is selected, and the ions will cause reduction or oxidation reaction of the colorant material to thereby allowing it to deposit (reduction) from its ionized status or to elute (oxidation).
Thus deposited metal can be observed from the substrate side so as to form a predetermined pattern since the reflected light of the incident light show a sufficient contrast between colors ascribable to the metal and the background. As a consequence, reversible reaction between coloring and decoloring makes it possible to configure a desired reflection-type element. It is also possible to configure a transmission-type element in which the incident light is observed from the substrate side as the transmitted light (the same will apply hereinafter).
In this case, a metal-ion-containing polyvinyl alcohol or the like is suitable for assisting ion conduction, and bismuth chloride is preferably used as the metal ion.
The ion-conduction-type display element not only makes it possible to solve the problems which have resided in the conventional display elements, and to thin the display element enough to an extent that the display device is available as the electronic paper, but makes it operable also under conditions under the simple matrix driving.
Next paragraphs will describe an exemplary configuration of the ion-conduction-type display element.
As shown in FIGS. 12A, 12B and 13, transparent electrodes 62 which comprise an ITO (indium tin oxide) film are formed on a transparent support 61 such as glass substrate by vapor deposition or sputtering, and by succeeding patterning. The individual pixel portions 57 composed of the transparent pixel electrodes 62 are arranged so as to have a dot or matrix pattern on the transparent support 61.
On the transparent support 61, also a polymer solid electrolyte layer 65 is formed. A synthetic resin which serves as a matrix (base) polymer of the polymer solid electrolyte layer 65 and a material for composing the electrolyte are mixed first, and white particles as a colorant material was further dispersed therein, to thereby prepare a liquid, and the liquid of the polymer solid electrolyte is coated.
A support 67 having opposing electrodes 66 already formed thereon is then bonded with the transparent support 61 having the transparent pixel electrodes formed thereon 62, so as to hold the liquid of the polymer solid electrolyte in between on the opposing electrode 66 side. The stack is then dried and allowed to gelate, to thereby complete a display element 68 having the polymer solid electrolyte layer 65 and having a matrix electrode 21 configured therein.
Next, using the ion-conduction-type display element 68, metal ions for developing colors are allowed to diffuse and migrate in the polymer solid electrolyte layer 65 (ion conductor) towards the pixel portions 57 of the opposing transparent pixel electrodes 62 under the electric field applied by an arbitrary opposing electrode 66 on the support 67, to thereby allow the metal ions to deposit through reduction on the transparent pixel electrode 62 as shown in FIGS. 12A, 12B and 13. In this process, an ion diffusion range A is defined by a range allowing the ions to migrate almost normal to the isoelectric plane between the electrodes 66 and 62, and widening from the electrodes 66 at an angle θ of approximately 45° (the same will apply also hereinafter).
It is, however, anticipated that turning ON of both of X2 electrode and Y2 electrode in the electrodes 66 and 62, respectively, and consequent activation of the electric field (potential difference) at the intersection may sometimes result in a partial overlapping between an ion diffusion range ascribable to the electrodes X2 and Y2 (indicated by meshed solid lines) and an ion diffusion range ascribable to the electrodes X1 and Y2 (indicated by dashed lines) depending on the pitch or the like of the electrodes 66, and this may cause a crosstalk portion 69 due to color mixing between the adjacent pixel portions 57, and may make it difficult to produce a sharp display. It is also anticipated that a pixel portion in the electric field activated between the X1 and Y2 electrodes, for example, may slightly be colored due to ions diffused from the adjacent X2 electrode.
The present invention was conceived after considering the aforementioned situations, and an object thereof is to provide a display element capable of ensuring a desirable coloring efficiency of the pixels, a sharp display without causing crosstalk, and simple matrix driving at low energies.